Cross-reference is made to co-pending U.S. patent application Ser. No. 07/800,339 to A. Sezginer et al. for "Nuclear Magnetic Resonance Pulse Sequences for Detecting Bound Fluid Volume," filed Nov. 27, 1991.
Nuclear magnetic logging tools, such as disclosed in U.S. Pat. Nos. 4,933,638 to Kenyon et al. for "Borehole Measurement of NMR Characteristics of Earth Formations, and Interpretations Thereof"; and 5,055,787 and 5,055,788 both to Kleinberg et al. for "Borehole Measurement of NMR Characteristics of Earth Formations", measure the number and nuclear magnetic resonance (NMR) relaxation rates of hydrogen atoms in the pore space of rocks by measuring the amplitude and decay rate of signals resulting from pulse-echo sequences. In essence, the nuclear magnetic logging tools send a stream of RF-pulses into the formation and monitor the returning pulses which are called spin echoes. The measurements made are typically cyclical, with each cycle taking up to several seconds. Interpretation algorithms are then used to find the formation properties of interest.
The signal measured by a nuclear magnetic logging tool, such as CMR, mark of Schlumberger (Combined Magnetic Resonance) tool, formerly the PNMT, mark of Schlumberger (Pulsed Nuclear Magnetism Tool) is proportional to the mean density of hydrogen nuclei in the fluid that occupies the pore-space. Since the hydrogen density in water and liquid hydrocarbons are approximately constant, the detected signal can be calibrated to give the volume fraction of the fluid occupying the pore space.
NMR relaxation of a water saturated porous rock is not a simple exponential relaxation. Pores of rocks are in a fast diffusion regime (Latour, L. L., R. L. Kleinberg and A. Sezginer, Journal of Coll. and Interf. Science, Vol. 150, No. 2, May 1992) where the NMR signal from each pore is approximately single-exponential, and the relaxation time is proportional to the volume to surface ratio of the pore. Several researchers have demonstrated for water saturated sandstones and for synthetic porous specimens that the pore size distribution is closely related to the distribution of NMR relaxation times. Furthermore, it has been shown that the distributions of spin-lattice relaxation time T.sub.1 and spin-spin relaxation time T.sub.2 are very similar, and the ratio T.sub.1 /T.sub.2 is in the relatively narrow range of 1.0 to 2.6 for sedimentary rocks. See Kleinberg et al., "T1/T2 Ratio and Frequency Dependence in Porous Sedimentary Rocks", Jnl of Colloid and Interface Science 158, 195-198 (1993). Therefore, distributions of both spin-lattice and spin-spin relaxation-times carry the same information, namely the distribution of volume to surface ratios of pores. For example, an inversion-recovery measurement (Farrar, T. C. and E. D. Becker, Pulse and Fourier Transform NMR, Academic Press, 1971) which reveals spin-lattice relaxation, will produce a signal, m(t), that is a superposition of relaxations at different rates: ##EQU1## where a(T.sub.1)dT.sub.1 is the volume fraction of the fluid whose spin-lattice relaxation time is between T.sub.1 and T.sub.1 +dT.sub.1.
Water that is bound to clay minerals, water in pores that are too small to be flushed by a feasible pressure gradient, and heavy (viscous) hydrocarbons all relax rapidly. Fluids that relax slowly have low viscosity and reside in large pores. Hence, the slowly relaxing fluids can be produced, that is, pumped to the surface, provided there is sufficient permeability. It has been shown that bound and unbound (producible) fluids can be distinguished by their relaxation times in water saturated rock samples. See C. Straley, C. E. Morriss, W. E. Kenyon, and J. J. Howard, "NMR in Partially Saturated Rocks: Laboratory Insights on Free Fluid Index and Comparison with Borehole Logs," presented at the annual SPWLA meeting, Midland, Tex., 1991. The volume fraction of unbound and bound fluids (UFV and BFV) and porosity .PHI. are related to the relaxation-time distribution function a(T1) as follows: ##EQU2## The cutoff relaxation time T.sub.c distinguishing bound fluids from unbound fluids is empirically determined to be 50 msec for spin-lattice relaxation and 33 msec for spin-spin relaxation for water saturated sandstones and for 100 psi capillary pressure. The cutoff poresize, hence the cutoff relaxation-time T.sub.c, that divides the bound and unbound fluids depend on the differential pressure applied to produce the fluid.
BFV and UFV add up to porosity .PHI. which is the total volume fraction of fluids in the rock that are observable by the NMR logging instrument. Hydrogen nuclei in the rock matrix and some of the clay-bound water relax too rapidly and are not detected by the CMR. Therefore, hydration water and a fraction of clay-bound water is not included either in the BFV or porosity measured by the CMR.
UFV is conceptually the same as the free fluid index (FFI) and BFV is conceptually similar to but not necessarily equal to the volume fraction of irreducible water, .PHI.S.sub.irr. Bound fluid volume and irreducible water can differ when BFV includes heavy oil or when it misses part of the clay-bound water because of limitations of the logging instrument.
The BVF, the volume fraction of rapidly relaxing fluid, can be measured faster than either the entire distibution function or porosity can be measured. Therefore, a rapid measurement that is tailored to give only BFV is less sensitive to motion of the logging tool. BFV can be logged at speeds that are standard in the industry (1800-3600 ft/hr). UFV can then be estimated by subtracting BFV from the porosity if porosity is known either from other logs or another logging pass of the CMR.